According to current theory, the smallest magnetic particle is the magnetic dipole, which as the name suggests, is a combination of a positive and a negative magnetic pole. The existence of a dipole is essentially due to the spins of the electrons possessed by an atom (i.e. dipole moment). The dipole moment of atoms that constitutes a material point in random directions to reduce free energy thereof, thus, exhibits no net magnetism.
The atomic dipoles constituted by the electron spins of certain elements, such as the ferromagnetic elements, significantly point in the same direction when an external magnetic field is applied thereto. The material is said to be magnetized and this is called spontaneous magnetization. A perfect alignment of the atomic dipoles of a ferromagnetic material only exists at absolute zero. The thermal energy of finite temperature causes the dipoles to fluctuate and therefore deviate from perfect alignment. It is a process of increasing entropy. The randomizing effect gets more important when temperature gets higher and may considerably reduce the intensity of magnetization. At the Curie point, the dipoles have completely random alignment. Above the Curie point, a ferromagnetic material behaves paramagnetically. During the phase transition, the spins of an atom of a material absorb a great amount of energy and the material takes in a great amount of heat. This is the basic principle that a thermal analyzer applies to detect the Curie point of a material.
The characteristic of abrupt change of heat absorbability is the essential feature of the Curie point. By making use of this feature, commercially available thermal analyzers are constructed to detect the Curie temperature. One of them is the DSC (differential scanning calorimeter) thermal analyzer built by Du Pont Co., U.S.A. The working principle of the DSC is to heat a test sample and monitor the change of the thermal property of the sample. An abrupt change marks, the temperature of the Curie point.
There are also other methods adopted to determine the Curie temperature. Some of them determine the Curie temperature by the change of magnetism or the change of electricity resulted therefrom, as described for example, in Japanese Patent No. 61-51580, Soviet Union Patent No. 1318948, and French Patent No. 2486661. As compared to the DSC method, most of these methods require complicated and elaborate equipment to conduct the measurement.
The DSC method, although simple in equipment and easy to use, has the disadvantage that it is not applicable to materials exhibiting no significant abrupt change of heat absorbability around the Curie temperature. This is perhaps due to the low magnetic anisotropy energy and weak spontaneous magnetization of these materials. To overcome this difficulty, an external magnetic field may be applied to the test sample to increase the magnetic anisotropy energy and to straighten the spontaneous magnetization of the sample.
It is therefore an object of the invention to provide a method and the instrument thereof to measure the Curie point of materials by using an external magnetic field, especially the Curie point of ferromagnetic materials possessing low magnetic anisotropy energy and weak spontaneous magnetization, and thus exhibiting no abrupt change of thermal property when tested with a conventional thermal analyzer.
It is another object of the invention to provide a method and the instrument thereof which can provide a precise measurement of the Curie point and also measure the increment of the magnetic anisotropy energy.
It is a further object of the invention to provide a method and the instrument thereof which applies an external magnetic field to the test sample of a conventional DSC test, and by increasing the intensity of the magnetic field, the identification of the Curie point can be improved.
There is, therefore, provided a method and the instrument associated therewith for determining the Curie temperatures of ferromagnetic materials which display no significant abrupt change in heat absorption around the Curie temperatures thereof by means of a conventional thermal analyzer with externally added facilities to provide an external magnetic field. The abrupt change of heat absorption of ferromagnetic materials is significantly increased by adding an external magnetic field to increase the magnetic anisotropy energy thereof and to strengthen the spontaneous magnetization thereof.
Because the present method and the associated instrument increase the significance of the abrupt change of thermal property of a DSC sample material, in the resulting experimental data from the DSC test it is clearer and easier to identify the Curie temperature.
The following examples only are preferred embodiments of the instrument of the present invention and the experimental results thereof, and are compared with the results obtained with the conventional DSC test, described with reference to the accompanying drawings.